1. Field of the Invention
The present invention relates to an amplifier circuit for video display. In particular, the present invention relates to an amplifier circuit for providing on screen a signal for a second image overlaying the video image.
2. Background of the Invention
On-screen Display (OSD) is a technique by which a signal for a second image is substituted for a video signal, so as to overlay the second image over the original video image. To provide the OSD image, the OSD signal is typically injected into an amplifier circuit for the video signal while, simultaneously, the video input signal to the amplifier circuit is attenuated or "blanked". To prevent the video image from showing through the OSD image, while the OSD signal is active, coupling of the video signal to the amplifier's output signal by feed through and crosstalk must be suppressed.
In the prior art, to suppress feed through and crosstalk, a complex electronic circuit which requires a large silicon area is used. Alternatively, a dedicated pin in a video driver circuit is provided for receiving a control signal which directs whether video or OSD signal is output from the video driver circuit. These methods are unsatisfactory because of the requirements of a larger die and a dedicated pin. In addition, these methods tend to degrade bandwidth, and are particularly to difficult to implment with an 8-volt power supply.